FIG. 1 illustrates a conventional Silicon Carbide (SiC) Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) 10. As illustrated, the SiC MOSFET 10 includes a p-type SiC substrate 12 which is conventionally 4H-SiC, a first n+ well 14 forming a source region of the SiC MOSFET 10, a second n+ well 16 forming a drain region of the SiC MOSFET 10, and a gate oxide 18 arranged as shown. A metal source contact 20 is formed over the first n+ well 14 to provide a source contact for the SiC MOSFET 10. Likewise, a metal drain contact 22 is formed over the second n+ well 16 to provide a drain contact for the SiC MOSFET 10. Lastly, a gate contact 24 is formed on the gate oxide 18. The gate contact 24 may be formed of polysilicon or a metal such as, for example, Aluminum (Al). In operation, when a positive gate voltage is applied to the gate contact 24, an n-type inversion channel is created between the n+ wells 14 and 16 forming the source and drain regions of the SiC MOSFET 10. When the gate voltage is greater than a turn-on, or threshold, voltage of the SiC MOSFET 10, current flows from the source region to the drain region of the SiC MOSFET 10.
One issue with the SiC MOSFET 10 is that it has low current when in the on-state due to: (1) low electron mobility in SiC in the range of less than 5 cm2V−1s−1 and (2) dangling bonds and surface traps near the interface of the SiC substrate 12 and the gate oxide 18. As a result, an on-resistance of the SiC MOSFET 10 is high. One approach to increase electron mobility in a MOSFET is to form an n-type (for n-channel MOSFET) counter-doped or buried channel at the surface of the substrate between the source and drain regions of the MOSFET, where the counter-doped channel is formed via ion implantation into the surface of the substrate or epitaxial growth (i.e., regrowth). However, while these conventional counter-doped or buried channels increase carrier mobility, they also substantially decrease the turn-on, or threshold voltage, of the MOSFET. Specifically, for the same counter-doped or buried channel thickness, the threshold voltage decreases as the doping concentration of the counter-doped or buried channel increases. As such, these conventional counter-doped or buried channels are not acceptable for high power SiC MOS devices, which must be normally-off devices (i.e., have significantly positive turn-on, or threshold, voltages).
Thus, there is a need for a MOS device, and method of fabrication thereof, that has high channel current when in an on-state while retaining normally-off behavior.